A STUDY OF MIXING AND HEAT TRANSFER IN COMPLEX CONFIGURATIONS USING ADVANCED RANS-BASED MODELSReynolds stress tensor

Scalar flux vector

Extensions following Sadiki et al. (2003) of the model by Sjörgen and Johansson (2000) for EARSM: , where is the curvature corrected vorticity tensor and model by Wikströmet al. (2000) for EASFM:

- Yun, A. Sadiki, J. Janicka
- Institute for Energy and Powerplant Technology
- Technical University of Darmstadt, Petersenstr. 30, 64287 Darmstadt, Germany
- Phone: +49615116-2533 email: yun_alexander@yahoo.de, www: http://www.tu-darmstadt.de/fb/mb/ekt

- Motivation
- 1. Many turbulent flow processes of engineering importance exhibit highly complex interacting phenomena, such as mixing, heat and mass transfer, chemical reaction, etc.
- 2. Single phase heat transfer enhancement techniques are well established for conventional and compact heat exchangers.
- 3. Some of the basic techniques used for passive enhancement include surface treatments, and entrance effects, flow distribution, and secondary flow.
- 4. Applicability of these techniques in microchannels, minichannels and minidevices requires a profound understanding of the complex mechanism of turbulent flow ongoing process.

4. Results and Discussion

4.1 Swirled flow with small swirl number

- Objectives
- To devise a complete, advanced EARSM/EASFM based model able to capture complex mechanismus of turbulent flows dominated by heat transfer and mixing process effects
- To validate and apply it to complex configurations:

a) In curved ducts of relevance in heat exchangers, cooling passages of gas turbines and automobile engines, turbulent flow and heat transfer give rise to the existence of so-called “camel back” shapes in the streamwise mean velocity and temperature distribution of the curvature.These shapes were not captured with existing linear, and nonlinear models as well as some RSM.

b) In swirled combustion flows, turbulent mixing are dominated by an extremely 3-D complex turbulent flow behavior of great interest for many combustion engines purposes.

Swirled flowConfiguration

Exp. Roback and Johnson (1983)

- METHODOLOGY
- Thermodynamically consistentmodeling based on the explicit algebraic formulation including:
- nonlinear pressure strain rate,
- anisotropy dissipation,
- curvature correction considerations.

Averaged governing equations and modeling

CONCLUSIONS

1. In curved ducts, the combination of advanced EARSM/EASFM allows to predict satisfactorily flow processes with heat transfer phenomena , in contrast to linear, nonlinear models and some models of second order level.

2. In swirled confined flows, turbulent flow and mixing processes dominated by the joint effects of geometry confinement (expansion ratio), circumferential velocity and swirl intensity (S) are satisfactorily described. Some results (turbulent scalar flux) are better than computations using second order level-models.

3. Dealing with configurations exhibiting strong wall effects, Low-Re approach is recommendable, as shown in this work. In such cases, LES can not be high performance.

- REFERENCES
- Johnson R. W., Launder B. E., 1985, Local Nusselt number and temperature field in turbulent flow through a heated square-sectioned U-bend, Int. J. Heat Fluid Flow 6, 171-180.
- Roback R., Johnson B.V., 1983, Mass and Momentum Turbulent Transport Experiments with Confined Swirling Coaxial jets, NASA Contractor Report 168252.
- Sadiki A., Jakirlic S., Hanjalic K., 2003, Towards a Thermodynamically Consistent, Anisotropy-Resolving Turbulence Model for Conjugate Flow, Heat and Mass Transfer. Turbulence, Heat and Mass Transfer 4. Begell House.
- Wallin S., 2000, Engineering turbulence modeling for CFD with a focus on explicit algebraic Reynolds stress models, Doctoral thesis, Norsteds Truckeri, Stockholm, Sweden.
- Wikstrom P.M., Wallin S., Johansson A.V., 2000, Derivation and investigation of a new explicit algebraic model for the passive scalar flux, Phy. Fluids, 12:688-702.

Applications, Results and Discussions

U-bend Duct Configuration

Exp. Johnson and Launder(1985)